Ultrasonographic diagnosis of humeral stress fracture in thoroughbred racehorses

Background: Humeral stress fractures in racehorses can progress to catastrophic fracture if unrecognized. Scintigraphy is the current gold standard diagnostic technique but is limited by accessibility and cost. Radiographs are inconclusive until sufficient bone modeling occurs to be visible. It was hypothesized that ultrasound could be used to visualize caudoproximal humeral stress fractures. Objectives: Examine horses with caudoproximal humeral stress fractures with ultrasound. Study Design: Clinical case series. Methods: Seven racehorses that had a clinical history consistent with the presence of a humeral stress fracture were examined using humeral ultrasound and radiography with or without scintigraphy from June 2013 through June 2021. Clinical and imaging findings are described. Results: Seven Thoroughbred racehorses aged 2-4 years had a history of acute onset of a severe lameness with 4 returning to training 3-12 months after layup for an unrelated reason. Nine of 10 humeral stress fractures (2 bilateral, 3 left, 2 right) were identified with ultrasound. Ultrasound abnormalities included a step defect (5 humeri, 5 horses), periosteal callus/roughening (7 humeri, 4 horses) and/or an abnormally convex contour of the caudal aspect of the humeral neck (6 humeri, 5 horses). Radiographs revealed periosteal (8 humeri, 6 horses) and/or endosteal (6 humeri, 4 horses) proliferation adjacent the caudoproximal aspect of the humeral cortex. Scintigraphy of 5 horses identified increased radiopharmaceutical uptake in the caudoproximal aspect of 7 humeri. Serial recheck radiography and ultrasound (5 horses) revealed bone remodeling. Horses were returned to intended use as racehorses (4) or riding horses (2) or were retired (1). Main Limitations: Small case series. Conclusions: Ultrasound is useful for caudoproximal humeral stress fracture detection and can be utilized to monitor healing.